1. Technical Field
The present invention relates to citrus peel waste processing and the recovery of high value material and chemicals. More specifically, the present invention relates to an improved sustainable method and system of processing citrus peels to remove limonene and pectin fractions thereby enhancing the remaining citrus peel solids for fermentation for the production of ethanol, valuable acids, methane and other recoverable high value chemicals resulting in a sustainable recovery method and system.
2. Description of Related Art
The generation of solid citrus peel waste presents a growing environmental and economic problem. Citrus peel waste consists mainly of citrus peels, membranes and seeds after the citrus fruit has been processed for the production of juices for consumption. It is estimated that approximately 5-7 billion pounds of citrus peel waste is produced annually from grapefruit and orange juice production operations in the State of Florida. Currently, the majority of this citrus peel waste is pressed and dried into cattle feed. At present prices, the large capital equipment and energy costs associated with the production and distribution of cattle feed from citrus peel waste are not recovered due to the low value of citrus waste as cattle feed and high cost of drying. Moreover, substantial amounts of essential citrus peel oil of d(+)-limonene are volatilized from the peel waste and often released into the atmosphere during peel drying operations in the feed production process. If the feed producer desires to recover this valuable limonene vapor fraction, they must install expensive vapor recovery systems to collect the limonene vapor.
Although citrus peel waste creates an environmental and waste disposal problem, it does contain substantial amounts of valuable chemical fractions such as soluble sugars and polymeric carbohydrates including pectin, cellulose and hemicellulose. These polymeric carbohydrates can be hydrolyzed to simple sugars by the use of acids or enzymes as known in the art. The resulting sugars may be fermented to ethanol, lactic acid or other products by known methods in the art.
One unique characteristic of citrus peels are that they contain a large amount of soluble sugars (e.g. glucose, fructose, galactose, arabinose and xylose) in solution within the tissues of the peels. By disrupting these tissues, the sugars can be released to increase the efficiency and speed of fermentation processes. Therefore, a need exists in the art for a process that effectively ruptures the citrus peel membrane to release the soluble sugars and other complex carbohydrates to improve the fermentation process at the highest concentrations possible. However, it is readily known that the presence of limonene in the citrus peel presents an obstacle to efficiently fermenting the citrus peels.
Limonene is a terpene contained within the peel of citrus fruits. Limonene is extremely toxic to fermenting microorganisms and its presence in the fruit peel provides a defense to mold and bacteria thereby naturally preventing microbial buildup and fermentation of the fruit. In processing peels into ethanol and/or other recoverable chemicals, it is desirable to remove a majority, if not all, of the limonene from the citrus peel prior to fermentation operations. For efficient fermentation, the limonene in the peel must be reduced below 3000 parts per million (ppm) and preferably below 1500 ppm as described and known in the art. Several methods of removing limonene from citrus peels are known such as cold pressing, supercritical CO2 extraction, steam stripping, direct heating in a dryer with subsequent condensation of limonene, and solvent washing. These methods while effective are energy and chemical intensive, expensive and typically cost prohibitive. As such, a need exists for an economical and efficient method that extracts a substantial amount of limonene from the peel prior to fermentation to provide for the efficient production of methane, ethanol and/or other desired high value chemical products without contributing additional chemical byproducts and volume to the waste stream.
It is recognized by industry that citrus peel waste is an excellent source of pectin, limonene and other polysaccharides. Following removal of these constituents, the remaining peel solids can be fermented and converted into high value end products such as methane and ethanol and thus reduce the volume of peel waste and reduce subsequent disposal costs. Various steps for accomplishing the recovery of these products from citrus peels are known in the art such as processes utilizing steam injection and fermenting sugars into ethanol. Nonetheless, conventional citrus peel treatment is energy intensive resulting in considerable power expenses for operation. The prior art does not disclose a system integrating the inventive methods disclosed herein for addressing this large energy requirement. As such, a need exists in the art for a sustainable citrus peel processing method that improves the process efficiency for the removal of limonene and pectin prior to the fermentation of the peel into ethanol, methane and other recoverable chemicals that can be operated and fueled from the end products generated from fermenting citrus peels.